The present invention relates broadly to a method and apparatus for thermal poling of materials and to devices incorporating poled materials.
The induced variation of the electro-optic (EO) coefficient of materials (hereinafter referred to as poling) has been attempted e.g. for optical fibres and bulk glass to produce a residual EO coefficient chi(2) in the glass material.
Two main methods are presently applied for poling optical fibres or bulk glass: (I) thermal poling and (II) ultraviolet (UV) poling. The latter is believed to effect poling through non-thermal effects caused by UV absorption in the glass.
In both methods, a high poling voltage is applied across the material during either the heating process or the UV absorption to produce the EO coefficient changes.
The largest values of the EO coefficient in glass have been produced by UV poling. However, the resulting EO variations have been difficult to reproduce and the underlying principles are not fully understood, which makes this method unsuitable for mass-production of poled materials.
Thermal poling involves the heating of the entire bulk glass or optical fibre in an oven. However, this method has been typically limited to uniform poling. For non-uniform poling, periodic electrodes have to be deposited onto e.g. the bulk glass.
This has required the heating to be performed in a vacuum to prevent smearing between adjacent poling domains by reducing electrical conductivity in air between the electrodes. This results in a complex poling system and furthermore, the periodic poling design of e.g. poled gratings was limited by the photolithographic mask used for the deposition of the electrodes. Furthermore, as the sign of the EO coefficient can only be changed by applying a poling voltage of different polarity, this is practically impossible with such a poling system, since at the high voltages required, shortening between adjacent electrodes would occur.
A first aspect of the present invention provides a method of thermally poling a silica-based waveguide, comprising the steps of:
exposing a region of the waveguide to an electric field;
directing a laser beam into the region which is exposed to the electric field;
irradiating the region at a power density selected to effect localised heating of the waveguide within the region through direct absorption of the laser radiation; and
scanning the laser beam over the region.
The method may further comprise scanning the laser beam across the region to effect poling of the region.
The method may comprise varying the power density of the laser beam while scanning. Accordingly, a method of non-uniform thermal poling can be provided.
A direction of the electric field may be changed as the laser beam is scanned over the region. Accordingly, it can be possible to alternate the sign of the EO coefficient in non-uniform thermal poling.
Where the material comprises glass, the laser beam is preferably an infrared (IR) laser, for example a CO2 laser.
Where the material is an optical fibre, wires may be inserted into tubular holes extending substantially parallel to a core of the optical fibre located between the tubular holes, and a differential voltage may be applied to the wires to create the electric field. The core of the optical fibre may comprise a germanosilicate material co-doped with phosphorous.
A second aspect of the present invention provides an apparatus for thermally poling a silica-based waveguide, comprising:
a means for exposing a region of the waveguide to an electric field;
a means for directing a laser beam into the region which is exposed to the electric field;
a means for irradiating the region at a power density selected to effect localised heating of the waveguide within the region through direct absorption of the laser radiation; and
a means for scanning the laser beam over the region.
A third aspect of the present invention provides an optical device incorporating a silica-based waveguide when thermally poled by the above-described method.
Preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings.